Vehicle engine systems may include various vacuum consumption devices that are actuated using vacuum. These may include, for example, a brake booster. Vacuum used by these devices may be provided by a dedicated vacuum pump, such as an electrically-driven or engine-driven vacuum pump. As an alternative to such resource-consuming vacuum pumps, one or more aspirators may be coupled in an engine system to harness engine airflow for generation of vacuum. Aspirators (which may alternatively be referred to as ejectors, venturi pumps, jet pumps, and eductors) are passive devices which provide low-cost vacuum generation when utilized in engine systems. An amount of vacuum generated at an aspirator can be controlled by controlling the motive air flow rate through the aspirator. For example, when incorporated in an engine intake system, aspirators may generate vacuum using energy that would otherwise be lost to throttling, and the generated vacuum may be used in vacuum-powered devices such as brake boosters.
Typically, aspirators are designed to maximize either vacuum generation or suction flow, but not both. Staged aspirators including multiple suction ports or taps may be used, but such aspirators tend to suffer from various disadvantages. For example, staged aspirators may rely on a motive flow of compressed air, and may not be usable in configurations where motive flow is intermittent (e.g., intermittent motive flow may result in vacuum reservoir vacuum loss in some examples). Further, staged aspirators may be configured such that suction flow must pass through multiple check valves en route to the suction port(s) of the aspirator, which may disadvantageously result in flow losses.
To address at least some of these issues, the inventors herein have identified a multiple tap aspirator which, when incorporated in an engine system, provides both high vacuum generation and high suction flow, and may be operated with a low pressure difference driving motive flow, during intermittent motive flow conditions. In one example, an engine system includes an aspirator with a suction tap in a throat of the aspirator, a suction tap in a diverging cone of the aspirator, and a suction tap in a straight tube downstream of the diverging cone. The inventors have recognized that placement of suction taps in the throat, diverging cone, and straight exit tube of the aspirator advantageously maximizes vacuum generation while enabling a high suction flow rate, in that this placement combines the advantages of throat tap aspirators (e.g., high vacuum generation) with the advantages of aspirators with taps arranged downstream of the throat (e.g., high suction flow). Inclusion of a tap in the exit tube (e.g., a straight, unconstricted tube downstream of the aspirator's diverging cone) advantageously enables fast pull-down of a vacuum source, such as a brake booster. Further, the inventors have recognized that such an aspirator may be powered by vacuum rather than compressed air. For example, a motive inlet of the aspirator may be coupled with atmosphere, and a mixed flow outlet of the aspirator coupled with a vacuum source, such that the pressure differential between atmosphere and the vacuum source induces flow through the aspirator. In other examples, the multiple tap aspirator may be coupled between an engine crankcase and a low pressure sink of the engine intake system such as the compressor inlet or intake manifold, so as to generate vacuum via crankcase ventilation flow. Furthermore, by including only a single check valve in the path between the source of suction flow and each suction tap of the aspirator, flow losses which often occur in staged aspirators featuring multiple check valves in the suction flow path may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.